The application generally relates to a control system for a compressor. The application relates more specifically to a capacity control system for a centrifugal compressor.
One method to control capacity in a centrifugal compressor is by adjusting the position of pre-rotation vanes (PRVs) or inlet guide vanes located at the inlet to the centrifugal compressor to control the flow of refrigerant or other fluid into the compressor. An actuator can be used to open the PRVs to increase the amount of refrigerant or fluid to the compressor and thereby increase the capacity of the system. Similarly, the actuator can be used to close the PRVs to decrease the amount of refrigerant or fluid to the compressor and thereby decrease the capacity of the system. Other methods that can be used to control capacity in a centrifugal compressor include adjusting the speed of the compressor with a variable speed drive or opening a hot gas bypass valve to direct a portion of the compressed refrigerant or fluid back to the suction inlet of the compressor. Each of these capacity control methods provides advantages and limitations.
A centrifugal compressor may encounter instabilities such as surge or stall during operation. Surge or surging is a transient phenomenon having oscillations in pressures and flow, and can result in complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. One technique to correct a surge condition can involve the opening of a hot gas bypass valve to return some of the discharge gas of the compressor to the compressor inlet to increase the flow at the compressor inlet. In contrast, stall or rotating stall is a local flow separation in one or more components of a compressor, and can have discharge pressure disturbances at fundamental frequencies less than the rotational frequency of the impeller of the compressor. Rotating stall in a fixed speed centrifugal compressor is predominantly located in the diffuser of the compressor and can be remediated with a variable geometry diffuser (VGD). The presence of rotating stall in the compressor can be a precursor of an impending surge condition.
One method for detecting and controlling rotating stall in a diffuser region of a centrifugal compressor includes using a pressure transducer placed in the compressor discharge passageway or the diffuser to measure the prevalent sound or acoustic pressure. The signal from the pressure transducer is filtered and processed via analog or digital techniques to determine the presence or likelihood of rotating stall. Rotating stall is detected by comparing a calculated energy amount from measured discharge pressure pulses or pulsations with a predetermined threshold amount corresponding to the presence of rotating stall.
However, for a portion of the operating range of a centrifugal compressor, the compressor can surge without the occurrence of a prior stall condition, especially when the compressor is operating at low speeds. When the compressor directly enters a surge condition, the control system for the compressor does not have an opportunity to sense for the precursor stall condition. Consequently, the control system of the compressor cannot initiate a corrective action for the stall condition to possibly avoid the onset of the surge condition. Other aspects of the control system for dealing with surge conditions in the compressor require that the control system identify a surge condition(s) and react in a predetermined sequence. For the control system to identify a surge condition, one or more surge cycles must occur during a predetermined length of time before the control system can take corrective action.
In many control systems, the capacity control function and the surge/stall control function or other control functions may be in conflict with each other. For example, the capacity control function may prescribe compressor operating conditions that can cause a surge or stall condition to occur. Similarly, the surge/stall control function may not provide the desired output capacity required by the capacity control function. In addition, other safety or operational parameters can further conflict with the capacity control function.
Therefore, what is needed is a system and method for controlling the capacity of a centrifugal compressor while maintaining stable operating conditions for the compressor.